Question

a characteristic of water vapor, but not other states of water, is
a. water molecules move about freely.
b. water vapor is compressible
c. small changes in the volume of water occur with changes in temperature
d. water exerts equal pressure on the sides of a container in which it is placed

Explanation:

• Option A: Molecules move freely in liquid (diffusion) and gas, so not unique to vapor.
• Option B: Gases (water vapor) have widely spaced molecules, making them compressible; liquids/solids have tightly packed molecules and are nearly incompressible. This is unique to water vapor.
• Option C: Liquid water has small volume changes with temperature; this is not unique to vapor.
• Option D: All fluids (liquid and gas water) exert equal pressure on container sides, so not unique to vapor.

Answer:

B. Water vapor is compressible